R(V)-DEPENDENT OPTICAL AND NEAR-ULTRAVIOLET EXTINCTION

We have derived extinctions A(lambda)/A(V) at the wavelengths of the uvby filters for 22 stars, with a range of values of R(upsilon), from the sample of Cardelli, Clayton, and Mathis (1989, hereafter CCM). We have fit these extinctions, and also UBVRIJHKL, IUE and ANS extinction measurements, with linear relations A(lambda)/A(V) = a + b/R(upsilon) and fit a and b as a function of x(= 1/lambda) with polynomials to obtain an R(u)psilon-dependent mean extinction law [A(x)/A(V) = a(x) + b(x)/R(upsilon)] in the optical and near-ultraviolet (1.1 mu m(-1) less than or equal to x less than or equal to 3.3 mu m(-1).) This law is virtually identical to the CCM extinction law for large values of R(upsilon) (R(upsilon) similar to 5) but is slightly lower in the near-ultraviolet for smaller R(upsilon) (R(upsilon) similar to 3). The extinction law presented here agrees much better with a high-resolution extinction curve for the diffuse interstellar medium (R(upsilon) similar to 3.1), presented by Bastiaansen (1992), than CCM. The deviations of individual extinction curves from the mean are dominated by observational errors. The wavelength resolution of this work is not high enough to show evidence for or against the existence of very broad structure in optical extinction curves.